Current sensing circuits using so-called current sensing transistors (or “sense FETs”) have been commonly used for years. Such current sensing techniques may be especially useful when measuring the load current of power field effect transistors (power FETs) which are composed of a plurality of transistor cells as illustrated, for example, in U.S. patent publication US 2001/0020732 A1. Such power field effect transistors have a common drain region for all transistors cells composing the power transistor component. The common drain region is connected by one drain electrode arranged on the back-side of a wafer whereas the source region and the respective source electrodes are contacted on the front-side of the wafer and connected in parallel. The source electrode of one transistor cell (referred to as “sense cell”) may be connected separately from the source electrode carrying the load current to tap a current signal (referred to as “sense current”) that is representative of the load current flowing through the remaining of the plurality of transistor cells which form the load transistor. Of course a few transistor cells may be connected in parallel to form the sense transistor.
In a circuit arrangement including a load transistor/sense transistor pair the source current of the sense transistor (i.e., the sense current) is directly proportional to the source current of the load transistor (i.e., the load current) whereby the factor of proportionality results from the ratio of the current conducting area of the load transistor and the current conducting area of the sense transistor which is (at least approximately) equivalent to the ratio of the number of transistor cells forming the load transistor and, respectively, the sense transistor.
The proportionality condition mentioned above is only met when both transistors (load transistor and sense transistor) exactly operate in the same operating point, i.e., when both transistors are supplied with the same gate-source voltage and are exposed to the same drain-source voltage. A number of circuitry is known which may be applied to ensure that both transistors operate in the same operating point. Just to give an example, for common drain MOS technologies an operational amplifier may be used to set the source-voltage of the sense transistor to match the source-voltage of the load transistor. Due to the common drain electrode an equal drain-source-voltage is achieved. Additionally, the gate electrodes of sense transistor and load transistor are connected so as to provide the same gate-source voltage to both transistors.
Although the sense transistor and load transistor operate in the same operating point due to appropriate circuitry, further undesired side effects and interactions between the both transistors may deteriorate the strict proportionality between the respective source currents. For example, the homogeneous drain current density is to be ensured throughout the transistor cells (of both transistors). An inhomogeneous drain current flow may result in internal transverse currents thus distorting the strict proportional relationship between the source currents of the sense transistor and the load transistor, respectively.
In view of the above there is a general need for improved current sensing circuit arrangements which (at least partially) solve or alleviate the problems arising when using known sense transistor circuits.